1. Field of the Invention:
This invention relates to a molded polyurethane article comprising a polyurethane foam core member and a skin layer formed integrally with said core member, and a method for producing said molded polyurethane article.
There are known a variety of products manufactured by thermoforming a polyvinyl chloride film into a predetermined shape, applying the shaped film to a foaming mold, pouring a foamable polyurethane composition into the mold and causing the polyurethane to foam and cure to yield a shaped article consisting of a foamed polyurethane core and and a polyvinyl chloride surface film as integrally joined thereto. These products are used in such applications as automotive and rolling stock cushions, pads, etc. This invention is not concerned with such products but with molded polyurethane articles, the skin layer of which is also made of polyurethane and a method of producing such polyurethane articles.
2. Prior Art:
A variety of molded polyurethane articles are known which have a skin layer also made of polyurethane. For example, a known process for producing such articles comprises depositing a hot melt polyurethane elastomer powder electrostatically on the internal surface of a mold, heating the mold to the fusing temperature of the elastomer to obtain a film, applying the film to a foaming mold (if necessary, a vacuum is applied to the film to bring it into intimate contact with the internal surface of the mold), pouring a foamable polyurethane solution into the formed film in the latter mold and causing the polyurethane to foam and cure. This method is however disadvantageous in that at the fusing and film-forming stage, the fused polyurethane elastomer flows at the flanged or deeply grained parts of the film-forming mold to leave only a thin film there.
There is also known a method which comprises spraying the inner surface of a film-forming mold with a two-pack type composition and causing the composition to cure. By this method, it takes several minutes to obtain a cured film. The corner of a mold is a line, though this may be an exaggeration, and even if it is actually a plane, it is a curved plane with a large radius of curvature. Therefore, at such linear or curved corner, or hanging surfaces, the composition flows away before it has been sufficiently cured until it is no longer able to flow in the absence of an external force, thus giving a very thin film. The above situation holds when the two-pack composition has been sufficiently diluted to an easily sprayable concentration or when the undiluted composition is applied by the airless high pressure spraying technique.
Another method, which has already been commercially applied, comprises preparing a coating composition from a sufficiently chain-extended polyurethane elastomer as the vehicle and a solvent and spraying the patterned inner surface of a mold with coating composition to produce a film. If a suitable solvent is chosen, this procedure yields a film having the desired thickness even in corner areas. This is because the elastomer which has been polymerized to a sufficiently high molecular weight is deposited on the inner surface of the mold so that only if the proper spraying technique, solvent and other conditions are selected, there is produced a film having a substantially uniform thickness even in corner areas.
However, since the polyurethane elastomer has a high molecular weight, it must be sufficiently diluted (a few to more than 10% concentration). Therefore, the large quantity of solvent must be employed and it results in high production costs, and moreover it is only possible to produce comparatively thin films (not more than 100 u). Thus, by any of the conventional methods it is not possible to produce a film having adequate strength, a uniform distribution of thickness and sufficient tenacity. Thus, it is not possible to obtain a desirable molded polyurethane article having a satisfactory skin layer by taking out such product film, with the desired shape retained, from a skin-forming mold, fitting the same film in a separate foaming mold and pouring a foamable polyurethane solution into the latter mold and causing the polyurethane foam and cure.